Integrated circuits are packaged at the individual die level. The integrated circuits are formed in the die of a semiconductor device wafer. During fabrication the die is covered with a passivation layer, typically silicon dioxide or silicon nitride, to protect the die during assembly and packaging. During assembly, the wafer is separated into a die by mounting it on an adhesive frame and sawing a plurality of cuts to separate the die from each other. A die attach machine removes the die from the frame and mounts it on a lead frame. The lead frame has a central die pad for supporting the die and a plurality of leads extending from the central die pad, typically one lead for each bond pad of the integrated circuit. The lead frame is passed through a wire bonding machine where a very fine conductive wire, typically aluminum or gold, is bonded from each bond pad to its corresponding lead on the lead frame. The wire bonded die is packaged in plastic. Plastic packaging is done in a molding operation where the wire bonded die is placed in a mold and molten plastic is injected into the mold. Ceramic packages are made by bonding the die to the lead frame in one-half of a ceramic shell and closing the shell with another ceramic half shell or lid. After packaging, lead frames are separated from each other and the leads are trimmed and bent into position. The packaged devices are assembled onto circuit boards with other devices where the different devices are interconnected to provide a system. So, assembly and packaging are both labor intensive and time-consuming, and have associated yield losses.
Conventional plastic and ceramic packages are many times the size of the die they protect. So, the packaging of the die occupies the majority of the space in any circuit board that interconnects integrated circuits. The overall size of computers and other electronic equipment would be greatly reduced if the packaged components were smaller. Smaller packages would be closer to one another, thereby reducing the size of metal traces that interconnect the integrated circuits on the circuit boards. With shorter traces to other circuits, the overall speed of the electronic system can be increased.
Integrated circuits often include micromachinable components such as trimmable resistors, fuses, and resonant beams. These components are machinable prior to application of a passivation layer or prior to encapsulation of the integrated circuit. However, there has developed a need for a micromachinable device that can be machined after packaging.
Integrated circuits often include components such as resonant beams, inductors, capacitors or air bridges which require hermetic cavities within the package. Plastic injection molded packages can not provide these hermetic cavities. Plastic packages with cavities will collect moisture that will damage the devices in the cavity.